Engines, including diesel engines, gasoline engines, gaseous fuel-driven engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous and solid material, including particulate matter, nitrogen oxides (NOx), and sulfur compounds.
Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent over the years. The amount of pollutants emitted from an engine may be regulated depending on the type, size, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of engine emissions is exhaust gas recirculation (“EGR”). Conventional EGR systems recirculate engine exhaust gas into the intake air supply of the engine. The exhaust gas directed to a combustion chamber in the engine reduces the concentration of oxygen within the combustion chamber and increases the specific heat of the air/fuel mixture, thereby lowering the maximum combustion temperature within the combustion chamber. The lowered maximum combustion temperature and reduced oxygen concentration can slow the chemical reactions forming NOx.
An EGR system for a spark-ignited engine is described in U.S. Pat. No. 6,758,195 (“the '195 patent”) to Jaye. The '195 patent describes an EGR system including an engine with a combustion chamber having a pair of intake ports, an exhaust port, and an EGR port. A pair of intake valves and an exhaust valve are actuated using a camshaft to allow flow through the respective intake and exhaust ports, and an EGR valve is separately actuated to allow flow through the EGR port. The EGR port is fluidly connected to the exhaust port to allow recirculated exhaust gas to flow to the combustion chamber.
Although the system of the '195 patent provides an EGR system, the EGR system is incorporated into a spark-ignited engine in which the timing patterns for the intake and exhaust valves are predetermined using a camshaft, and the EGR valve is opened after the intake valve is opened. This is sufficient for spark-ignited engines in which the intake air is typically at a lower pressure than the recirculated exhaust gas. Intake air having a lower pressure is supplied first to the combustion chamber, and then recirculated exhaust gas having a higher pressure is supplied. However, when the recirculated exhaust gas has a lower pressure than the intake air and the intake air is supplied to the combustion chamber first, the higher pressure intake air may attempt to flow out of the combustion cylinder through the EGR port after the EGR valve is opened. As a result, there may be a backflow of intake air through the EGR port, and this backflow reduces the efficiency of the EGR system, thereby reducing the ability of the EGR system to reduce exhaust emissions effectively. Alternatively, there may be no flow of recirculated exhaust gas into the combustion chamber so that the efficiency of the EGR system would be less than zero and not just low. Furthermore, in the EGR system of the '195 patent, the recirculated exhaust gas flows directly from the exhaust port to the EGR port. Therefore, the EGR system of the '195 patent does not allow for treatment of the recirculated exhaust gas before the recirculated exhaust gas is supplied to the EGR port.
The disclosed system is directed to overcoming one or more of the problems set forth above.